1. Field of the Invention
The present invention relates to an ultrasonic diagnostic scanner which extracts a harmonic component caused by ultrasonic nonlinear propagation in biological tissues to image the cross section of the biological tissues.
2. Description of the Related Art
Known diagnostic ultrasonography has often used a technique of imaging the cross section of biological tissues using a fundamental contained in echo signals from the biological tissues. However, the technique of using the fundamental of echo signals often generates artifacts, thus producing the problem of decreasing the quality of the diagnostic images.
Accordingly, tissue harmonic imaging has recently been used in which a detailed cross-section of tissue structure is imaged using the nonlinearity of the ultrasonic propagation speed in biological tissues.
The tissue harmonic imaging is a technique of imaging the cross-section of biological tissues using only the second harmonic of the harmonic component in echo signals from biological tissues, and features clear high-contrast imaging with reduced artifacts.
This has made an improvement in the image quality of diagnostic images in the present diagnostic ultrasonography, thus improving the performance of diagnostic ultrasonography.
A known method for extracting only a harmonic component is pulse inversion (PI) imaging (for example, Abiru Iwao, and Kamakura Tomoo, Cyouonpa parusu no hisenkeidenpan [Unlinear Propagation of Ultrasonic Pulse], Singakugihou, US 89-23, p. 53). In this pulse inversion imaging, two kinds of ultrasonic waves with inverted phases are sent to multiple scanning lines, and two echo signals corresponding to the two ultrasonic waves are received. Then only harmonic components are extracted from the biological tissues by adding the echo signals and removing fundamental components.
In the tissue harmonic imaging, only difference tones of the harmonic components contained in the echo signals from biological tissues are used to image the cross-section of the biological tissues (for example, refer to JP-A-2004-298620).
Although it is not a technique of imaging the cross-section of the biological tissues, a technique of imaging the dynamic behavior of blood has also been used by using the fact that contrast medium bubbles are extremely delicate.
A known method for extracting only echo components from contrast medium bubbles includes rate subtraction (RS) imaging (for example, refer to JP-A-8-336527). In this rate subtraction imaging, the same ultrasonic wave is sent to multiple scanning lines at high sound pressure two times and two echo signals corresponding to the two times of transmission are received. Then the two echo signals are differentiated to remove duplicate components, so that an echo component from the disappeared and deformed contrast medium bubbles is extracted.
In other words, since the contrast medium bubbles for use in diagnostic ultrasonography are very delicate, when ultrasonic waves are applied, most of the bubbles are broken instantly. Accordingly, the echo signal obtained by the second ultrasonic transmission becomes smaller than that by the first transmission. However, the echo signal from the biological tissue does not change significantly. Accordingly, the differential signal obtained by the two echo signals reflects the echo signal from the disappeared and deformed contrast medium bubbles. Thus, the use of the rate subtraction imaging removes the echo signal from the biological tissues to enable imaging of only the dynamic behavior of blood.
However, the tissue harmonic imaging has the problem of insufficient sensitivity at the depths of ultrasonic images in comparison with the technique of imaging the cross-section of biological tissues from the fundamental in echo signals.